Method for treating or pre-treating components comprising aluminum surfaces

ABSTRACT

The invention relates to a method for treating or pre-treating parts, profiles, strips, or wires comprising surfaces of aluminium, or alloys containing aluminium optionally in the presence of surfaces consisting of other metals or alloys with an acidic, aqueous solution containing fluoride and phosphate. Said method is characterised in that the fluoride is present at least partially as a free fluoride in the solution and that in the bath of phosphating solution, the content of free fluoride is maintained within a concentration range of between 5 and 500 mg/l F free  and that of aluminium is maintained within a concentration range that is less than or equal to 100 mg/l Al ions (including complexed Al). To achieve this, increases in the aluminium content are reduced to contents of less than or equal to 100 mg/l Al ions in the bath, using a precipitation container outside the phosphating bath by circulating the phosphating solution from the phosphating bath to the precipitation container and vice versa.

[0001] The invention relates to a method for the treatment orpretreatment of parts with aluminium surfaces.

[0002] Phosphatizing methods for aluminium and aluminium alloys areknown in principle. In industrial practice, fluoride-modifiedphosphatizing methods with at least 150 mg/l free fluoride have provedto be particularly successful. These methods are important in particularin the automotive industry and are chiefly used when a mix of substratesof various metals or alloys is passed through the plants.

[0003] However, these methods have the serious disadvantages that due tothe high fluoride content comparatively large amounts of cryolite(Na₃AlF₆) and/or related precipitates are precipitated in thephosphatizing bath, and that parts of the precipitates are alsoprecipitated on the phosphatized surface without being removabletherefrom by means of simple rinsing operations, with an increasedroughness being produced on the pretreated surface which, even after thesubsequent application of lacquer, has a disturbing effect with itsroughness.

[0004] DE-A1-197 35 314 describes a method for the pretreatment ofcomponents with aluminium surfaces—if applicable in the presence ofmagnesium, steel and/or zinc surfaces—in a phosphatizing plant in whichthe components are degreased by means of a degreasing solution, arephosphatized by treatment with a phosphatizing solution containingfluoride, and are subsequently passivated by treatment with apassivating solution. The proportion of the aluminium and/or magnesiumsurface with respect to the entire surface of the components to betreated is at least 10% in this connection. The fluoride should be addedto the phosphatizing solution exclusively as complex-bound fluoride, andthe free-fluoride ion content formed therefrom in the phosphatizingsolution for phosphatizing the steel and/or zinc surfaces withoutphosphatizing the aluminium and/or magnesium surfaces should bemaintained at less than 100 mg/l. The passivating solution should alsobe composed in such a way that it passivates the phosphatized steeland/or zinc surfaces and forms a conversion layer on the aluminiumand/or magnesium surfaces.

[0005] However, this method has the disadvantage that it can only beused with comparatively small surface proportions in terms of aluminiumsurfaces, mostly only up to approximately 20% by surface of all thesurfaces to be treated, with respect to the mix of substrates. Inaddition it has the disadvantages that cryolite and/or relatedprecipitates are still formed in the pretreatment bath, and that with anincreased aluminium content of the phosphatizing solution thelayer-forming reactions, in particular on iron and steel surfaces, areimpaired so that the entire mix of substrates of various metal and alloysurfaces can no longer be coated well in the bath in a uniform manner.

[0006] It is an object of the invention to overcome the disadvantages ofthe prior art and propose in particular a method for phosphatizingaluminium and alloys containing aluminium, which method, even withincreased proportions of aluminium-containing surfaces of the componentsto be treated or to be pretreated, allows the application of a goodconversion layer that is also applicable on an industrial scale and/or acorresponding passivation layer on the surfaces of aluminium or alloyscontaining aluminium—if applicable in a mix of substrates of variousmetals or alloys.

[0007] The object is achieved by means of a method for the treatment orpretreatment of parts, sections, strips or wires with surfaces ofaluminium or alloys containing aluminium—if applicable in the presenceof surfaces of further metals or alloys—with an acid aqueous solutioncontaining fluoride and phosphate, which method is characterised in thatthe fluoride is at least partly present in the solution as freefluoride, and in that, in the bath of the phosphatizing solution

[0008] the free-fluoride content is maintained at a concentration in theregion from 5 to 500 mg/l F_(free), and

[0009] the aluminium content is maintained at a concentration in theregion of ≦100 mg/l Al ions (including complex-bound Al)

[0010] by virtue of the fact that increasing aluminium contents, in aprecipitation tank outside the phosphatizing bath, are decreased tocontents ≦100 mg/l Al ions in the bath by circulating the phosphatizingsolution from the phosphatizing bath to the precipitation tank and back.

[0011] Alternatively or at the same time, it can be ensured in the caseof the method in accordance with the invention that in a separate zoneof the phosphatizing bath increasing aluminium contents in thephosphatizing solution are decreased to contents ≦100 mg/l Al ions.

[0012] As a result of the pickling attack, aluminium is solubilized atthe metallic surface, and an aluminium content is absorbed into thephosphatizing solution. A certain aluminium content may, however, alsoget into the phosphatizing solution from other sources, for example fromchemicals introduced into the rinsing solution. The aluminium contentcan mainly be decreased or completely be decreased by precipitation, butin part also by complex formation. The content of free fluoride in thephosphatizing solution is preferably maintained in a range from 6 to 120mg/l, in particular preferably in a range from 10 to 80 mg/l, andespecially preferably in a range from 20 to 50 mg/l. The aluminiumcontent in the phosphatizing solution is preferably maintained at values≦80 mg/l, in particular preferably ≦60 mg/l, and especially preferably≦30 mg/l.

[0013] The method in accordance with the invention is distinguished byvirtue of the fact that in the precipitation tank or in the separatezone of the bath aluminium is precipitated in the phosphatizing solutionby adding alkali ions, fluoride complexes and/or fluoride ions, inparticular by means of Na or K ions or by means of at least one easilydissociating fluoride such as, for example, NaF, NH₄F, NaHF₂ or KF. TheAlF_(x) complex may be present in an already precomplexed form in thiscase. It is advantageous to control the method in accordance with theinvention in such a way that despite the addition of F the content offree fluoride in the bath is not increased. The alkali ion contents inthe phosphatizing bath preferably amount to 1 to 20 g/l and arepreferably also maintained in this range, in particular in the rangefrom 3 to 10 g/l. In principle, the alkali ion content may also lie farabove the concentration of 20 g/l, for example at 30 g/l. In many cases,however, given such a high alkali concentration, instability of the bathmay occur. In the case of a mixture of various alkali ions one besidethe other, a dominant sodium and/or potassium ion content is preferred.

[0014] This leads to the fact that the precipitated cryolite and therelated precipitates, such as K₂NaAlF₆, for example, do not occur orhardly occur any more in the phosphatizing bath, but largely or (almost)exclusively in the separate zone of the bath or in the precipitationtank. As a result, the precipitates can no longer settle or hardlysettle any more on the surface to be coated.

[0015] Advantageously, the free-fluoride concentration of thephosphatizing solution in the phosphatizing bath is 8 to 80 mg/l and inparticular is 10 to 50 mg/l; and the concentration in the precipitationtank or in the separate zone of the bath container is 5 to 500 mg/l freefluoride, in particular 20 to 200 mg/l, and especially preferably 30 to120 mg/l. It is therefore also preferable to maintain these contents atthese concentrations. Since below 5 mg/l usually there is no picklingattack against aluminium by free fluoride, and since this picklingattack is needed, as the subsequent passivation for example alone bymeans of chromate-containing compounds, titanium fluoride, zirconiumfluoride, a soluble rare-earth compound—in particular acerium-containing compound, where scandium, yttrium and lanthanum areincluded by the term rare-earth element—on the basis of silane,self-organizing molecules on the basis of phosphonate, a polymer solubleand/or dispersible in solvent is not sufficient for passivation, a smallminimum free-fluoride content in the phosphatizing solution is required.On the other hand, in many cases a closed phosphate layer is formed onthe metallic surfaces from the phosphatizing bath at contents above 120mg/l free fluoride, if the content with respect to cationsco-precipitated with the phosphate, such as Zn, Cu, Ni, Fe, Mn etc., isnot very small. The formation of a phosphate layer on the aluminiumsurfaces for reasons of protection against corrosion is not absolutelynecessary. Therefore, a free-fluoride content above 120 mg/l will notusually be chosen, although in accordance with the invention it is alsopossible to operate above this value, because it causes additionally ahigher consumption of chemicals and a larger amount of precipitatedsludge of cryolite and/or related precipitates. Moreover, a differencein the free-fluoride concentration between the phosphatizing bath andthe precipitation tank or separate zone in the bath of 30 to 60 mg/lshould preferably be adjusted and maintained.

[0016] In many cases, the method in accordance with the invention willbe utilized in such a way that the dwell time of the phosphatizingsolution in the precipitation tank or in the separate precipitation zoneis up to 1 h, often up to 0.5 h. The volume flow from the bath to theprecipitation tank and back is adjusted in accordance with the chosenvolumes or partial volumes as well as the desired aluminium content inthe phosphatizing bath.

[0017] The term “part” for the purposes of this application includes allkinds and shapes of sheets, strips and sections, moulded articles,semifinished products, components, assemblies etc.

[0018] In the case of the method in accordance with the invention, theparts, sections, strips and/or wires to be treated or pretreated areusually cleaned, rinsed and, if appropriate and separately from therinsing and cleaning stages, brought into contact with an activatingsolution, for example on the basis of colloidally dispersed titaniumphosphate, prior to pickling/phosphatizing.

[0019] After pickling/phosphatizing, the treated or pretreated parts,sections, strips and/or wires can be rinsed and/or passivated, inparticular by means of a passivating solution on the basis of achromate-containing compound, titanium fluoride, zirconium fluoride, asoluble rare-earth compound—in particular a cerium-containing compound,self-organizing molecules, for example on the basis of phosphonate, onthe basis of silane, a polymer soluble and/or dispersible in solvent.

[0020] After pickling/phosphatizing or after passivation, the treated orpretreated and/or passivated parts, sections, strips and/or wires may bedried. In some cases, for example in the case of immediately followingelectro-dipcoating, drying is not, however, required.

[0021] The precipitation of the aluminium may be effected under normalpressure and at a temperature in the range from room temperature to 70°C., in particular at a temperature in the range from 40 to 60° C. In thecase of the method in accordance with the invention, the formation ofthe conversion or passivation layer may be effected under normalpressure and at a temperature from room temperature to 70° C.,preferably at 35 to 60° C. The pH value usually lies in the range from 2to 4. In principle, the pH value of phosphatizing baths always lies inthe range around pH 3. At values of pH ≧4.0, the bath is usuallyunstable, whilst at values of pH ≦2.0 the bath is so stable that usuallythere is no good formation of layers, because the displacement of the pHvalue at the freshly pickled metallic surface is not sufficient fordeposition of the conversion layer.

[0022] Finally, the treated or pretreated and/or passivated parts,sections, strips and/or wires can be coated with a lacquer, with anotherkind of organic coating, with a film and/or with an adhesive layer, ifapplicable printed and if applicable reshaped, where the metal partscoated in this way can in addition be bonded, welded and/or otherwiseconnected together with other parts.

[0023] The products produced in accordance with the invention can beused in the automotive industry, in the aeronautical industry, inapparatus and machine construction, in the furniture industry, in thebuilding trade, for household appliances, electrical appliances,measuring instruments, control devices, testing devices, constructionelements, housings, panellings, shelf systems, racks, frames, dividers,partitions, trim panels, lighting fixtures, crash barriers, radiator orfence elements as well as small parts, in particular for car body partsor car bodies.

[0024]FIG. 1 shows a flow diagram in which one possible principle out ofseveral principles of the separate aluminium deposition is representedschematically.

[0025] Compared with methods described and practised so far, thephosphatizing method in accordance with the invention has the advantagethat the sludge with its cryolite content and/or related precipitatescontent is largely obtained in a separate precipitation zone or in aseparate precipitation tank and can be disposed of from there. With themethod in accordance with the invention it is also possible to treat orpretreat different metallic substrates in one mix, without the formationof layers, for example on steel, being impaired as a result. On accountof the reduced free-fluoride contents in the phosphatizing solution,there is also a reduced pickling attack against aluminium-containingsurfaces, which also gives rise to a correspondingly reduced formationof sludge. It has also been possible to ensure by means of the method inaccordance with the invention that, on account of only few particlesbeing co-precipitated and deposited on the phosphatized surface afterthe subsequent application of lacquer, no disturbing markings such asroughness, streaking or other irregularities could be detected.

[0026] Surprisingly, largely stable bath conditions with respect to thefree-fluoride content and aluminium content could be realized in thephosphatizing bath as well as in the separate zone or in theprecipitation tank despite the very different concentrations of thesecontents.

[0027] The subject-matter of the invention will be explained in greaterdetail in the following with reference to an exemplifying embodiment.

EXAMPLE

[0028] The following experiments were performed in order to determinethe suitable precipitation conditions for aluminium-containingphosphatizing solutions in a phosphatizing bath in accordance with theinvention. Above all, the influences of the concentrations of freefluoride, sodium as well as complex-bound fluoride on the speed of theprecipitation of aluminium were examined in this connection.

1. Test Design

[0029] A computer-assisted test design was employed in order to be ableto recognize in a better way interactions of the significant parametersthat might possibly exist. The Stavex 4.3 program was used.

2. Analyzers

[0030] Fluoride measurement: Orion Model 960 with ion-selectiveelectrode

[0031] Aluminium ICP.

[0032] A standard phosphatizing solution was prepared from p.a.chemicals to have following composition:

[0033] Zn: 1.5 g/l

[0034] Mn: 1.0 g/l

[0035] Ni: 1.0 g/l

[0036] P₂O₅: 14.0 g/l

[0037] NO₃: 3.0 g/l

[0038] SiF₆: 1.0 g/l

[0039] FA: 2.0 free acid.

[0040] Proceeding from this solution, ions of SiF₆ and Na as well as theproportion of free fluoride were varied in further preparations.

3. Practical Execution

[0041] 0.5 l of the phosphatizing solution described above was pouredinto a plastics beaker, and the possibly desired higher silicofluoridecontent was adjusted by means of a 24% H₂SiF₆ solution, the correctionof the free-acid content being effected by adding NaOH. The Na contentwas adjusted by means of NaNO₃; the corresponding free-fluoride contentwas adjusted by means of a dilute ammonium bifluoride solution. 20 mg/laluminium were added to the bath by means of a dilute aluminium nitratesolution. After 0, 15, 30 and 60 minutes, the free fluoride wasmeasured, and a sample of the solution was discharged for thedetermination of aluminium.

[0042] To prevent postprecipitation of the aluminium in the sampledrawn, the procedure was as follows: 5 ml of the sample filtered bymeans of a membrane filter was added to a strongly hydrochloricdeionized-water solution and filled up to 50 ml by means of deionizedwater. This solution was examined for its aluminium content by means ofICP.

[0043] The results are compiled in Table 1. TABLE 1 Precipitation testsContent of F_(free) in the Content of Al in the F_(free) Na SiF₆phosphatizing solution (mg/l) phosphatizing solution (mg/l) Test (ppm)(g/l) (g/l) 0 min 15 min 30 min 60 min 0 min 15 min 30 min 60 min 5 days1 40 3.0 1.00 39 25 27 27 20 20 20 20 20 2 80 3.0 1.00 78 53 52 58 20 2019 19 3.5 3 40 7.0 1.00 39 28 27 28 20 19 19 19 4 4 80 7.0 1.00 80 50 4644 20 1.5 1 0.5 5 40 3.0 3.00 46 44 44 44 20 19 19 19 20 6 80 3.0 3.0079 19 36 54 20 19 18 19 2 7 40 7.0 3.00 40 16 17 22 20 19 20 19 2 8 807.0 3.00 79 54 48 148* 20 1 0.5 <0.5 9 60 5.0 2.00 60 34 40 106* 20 104.5 2 10 43 5.0 2.00 46 25 34  80* 20 18 11 5 11 77 5.0 2.00 73 67 42 3720 19 17 13 12 60 3.3 2.00 59 43 39 28 20 19 19 19 3 13 60 6.7 2.00 5952 51 33 20 15 7.5 3.5 14 60 5.0 1.15 61 32 25 21 20 19 2 1 15 60 5.02.85 59 29 25 27 20 4 2 1

[0044] In Test 1 no precipitation of aluminium was observed over theperiod of 5 days. Tests 4, 8 and 15 revealed a drastic reduction in thealuminium content even within 15 minutes. Increased alkali contents arepreferred in conjunction with increased free-fluoride contents. SiF₆ wasadded because of the stability of the phosphatizing solution and forusability for a mix of various kinds of metallic surfaces, in particularto avoid stippling on zinc-plated or zinc-containing surfaces.

[0045] Upon evaluation of the values found in Example 1, the followingexemplifying calculation can be made:

4. Exemplifying Calculation of the Volume Flows or of the ReactionVessel

[0046] Phosphatizing bath 200 m³ Partial stream for precipitation 100 m³in a precipitation tank Feed of Al per hour 10 ppm

[0047] This feed corresponds to about 40 car bodies per hour with analuminium-surface proportion of 50% and a pickling attack ofapproximately 1 g/m² during the contact time.

[0048] Calculation formula for the stationary concentration of aluminiumin the phosphatizing solution:

[0049] Al(ppm)=(((200 m³−partial stream)×conc. Al in ppm after 1h+(partial stream×rest Al in ppm after precipitation))/200)+Al in ppmfrom feed via pickling attack/h.

[0050] Hence it was concluded that it is necessary for the volume flow,which is to be treated, to be at least 0.5 basin volumes per hour inorder to be able to detect a sufficiently high proportion of dissolvedaluminium. With an average dwell time of 30 min, this results in avolume of the reaction bath of 50 m³. A residual aluminium content inthe outlet of the treatment bath of approximately 5 ppm results in atheoretical stationary aluminium content in the treatment bath ofapproximately 25 ppm. However, since a partial precipitation of thealuminium can also be expected in the treatment bath (see for exampleTest 3 from Table 1), the actual content in terms of dissolved aluminiumwill adopt a lower value. Approximately 20% of all the precipitates areobtained in the phosphatizing bath and provided an Al content ofapproximately 20 ppm in the phosphatizing solution of the bath in thestationary condition. Approximately 80% of all the precipitates wereprecipitated in the precipitation tank.

1. Method for the treatment or pretreatment of parts, sections, stripsor wires with surfaces of aluminium or of alloys containing aluminium—ifapplicable in the presence of surfaces of further metals or alloys—withan acid aqueous solution containing fluoride and phosphate,characterised in that the fluoride is at least partly present in thesolution as free fluoride, and in that, in the bath of the phosphatizingsolution, the free-fluoride content is maintained at a concentration inthe range from 5 to 500 mg/l F_(free), and the aluminium content ismaintained at a concentration in the range of ≦100 mg/l Al ions(including complex-bound Al) by virtue of the fact that increasingaluminium contents, in a precipitation tank outside the phosphatizingbath, are decreased to contents ≦100 mg/l Al ions in the bath bycirculating the phosphatizing solution from the phosphatizing bath tothe precipitation tank and back.
 2. Method according to claim 1,modified in that, in a separate zone of the phosphatizing bath,increasing aluminium contents in the phosphatizing solution aredecreased to contents ≦100 mg/l Al ions.
 3. Method according to claim 1or 2, characterised in that, in the tank or in the separate zone of thebath, aluminium is precipitated in the phosphatizing solution by addingalkali ions, fluoride complexes and/or fluoride ions, in particular bymeans of Na or K ions or by means of at least one easily dissociatingfluoride such as, for example, NaF, NH₄F, NaHF₂ or KF.
 4. Methodaccording to one of the preceding claims, characterised in that thealkali ion content in the bath is maintained at a concentration in therange from 1 to 20 g/l.
 5. Method according to one of the precedingclaims, characterised in that the free-fluoride concentration in theprecipitation tank or in the separate zone of the bath container is 5 to500 mg/l free fluoride.
 6. Method according to one of the precedingclaims, characterised in that the dwell time of the phosphatizingsolution in the precipitation tank or in the separate precipitation zoneis up to 1 h.
 7. Method according to one of the preceding claims,characterised in that, prior to pickling/phosphatizing, the parts,sections, strips and/or wires to be treated or pretreated are cleaned,rinsed and, if appropriate and separately from the rinsing and cleaningstages, brought into contact with an activating solution, for example onthe basis of colloidally dispersed titanium phosphate.
 8. Methodaccording to one of the preceding claims, characterised in that, afterpickling/phosphatizing, the treated or pretreated parts, sections,strips and/or wires are rinsed and/or passivated, in particular by meansof a passivating solution on the basis of a chromate-containingcompound, titanium fluoride, zirconium fluoride, silane, self-organizingmolecules for example on the basis of phosphonate, a polymer solubleand/or dispersible in solvent, a soluble rare-earth compound—inparticular a soluble cerium-containing compound, with the termrare-earth element also including scandium, yttrium and lanthanum. 9.Method according to one of the preceding claims, characterised in thatthe treated or pretreated and/or passivated parts, sections, stripsand/or wires are dried after pickling/phosphatizing or afterpassivation.
 10. Method according to one of the preceding claims,characterised in that the precipitation of the aluminium is effectedunder normal pressure and at a temperature in the range from roomtemperature to 70° C.
 11. Method according to one of the precedingclaims, characterised in that the formation of the conversion orpassivation layer is effected under normal pressure and at a temperaturefrom room temperature to 70° C.
 12. Method according to one of thepreceding claims, characterised in that the treated or pretreated and/orpassivated parts, sections, strips and/or wires are coated with alacquer, with another kind of organic coating, with a film and/or withan adhesive layer, if applicable printed and if applicable reshaped,where the metal parts coated in this way can in addition be bonded,welded and/or otherwise connected together with other parts.
 13. Use ofthe products produced in accordance with the method according to claims1 to 12 in the automotive industry, in the aeronautical industry, inapparatus and machine construction, in the furniture industry, in thebuilding trade, for household appliances, electrical appliances,measuring instruments, control devices, testing devices, constructionelements, housings, panellings, shelf systems, racks, frames, dividers,partitions, trim panels, lighting fixtures, crash barriers, radiator orfence elements as well as small parts, in particular for car body partsor car bodies.